The present invention relates to a transmission for a hybrid automotive vehicle that reduces the sound level of a hydraulic pump driven by the transmission, to a method for estimating the noise level of a hydraulic machine, and to a hybrid automotive vehicle comprising the transmission.
A known type of transmission for a hydraulic hybrid automotive vehicle is described in particular in document FR-A1-2973302. The transmission disclosed therein has a first input connected to a combustion engine and a second input connected to a hydraulic pump. The motor and pump are aligned on the same axis and disposed on either side of a planetary gear set integrated in the transmission. This transmission further comprises a third input connected to a hydraulic machine, and an output through a differential to provide power to the two driving wheels of the vehicle.
The hydraulic machine can be connected to the differential through two different gear ratios, selectable by means of a first synchronizer sleeve.
The outer ring gear of the planetary gear set, arranged along the axis of the combustion engine, is driven by the combustion engine and comprises a sun gear directly connected to the hydraulic pump. The planet carrier of the planetary gear set can be connected to the sun gear by a second synchronizer sleeve in order to lock the planetary gear set, so that all of its elements rotate at the same speed. The planet carrier can also be connected to the output differential by a third synchronizer sleeve.
The hydraulic machine and the pump are connected to pressure accumulators which store energy when the pump delivers a braking torque and which return the energy to the hydraulic machine in order to deliver engine torque to the driving wheels.
When the planet carrier is locked, the pump runs at the same speed as the combustion engine. In another operating mode, when the planet carrier functions as a differential, the pump rotates in the opposite direction relative to the combustion engine and generates a braking torque. The pump rotates at a speed which may be lower or higher than the engine speed.
When the pump operates at a relatively high rotational speed and has a cylinder displacement calculated based on this speed, the pump receives mechanical power depending on the product of speed and displacement, which is used to generate the hydraulic pressure for recharging the pressure accumulators.
In addition, a known type of hydraulic pump comprises cylinders arranged parallel to the axis and distributed around this axis and comprises pistons actuated by a plate driven in rotation and capable of tilting around an axis perpendicular to the axis of rotation.
A problem with the above described type of pump used for hybrid vehicles is that at these speeds the pump generates unpleasant vibrations and loud noises. In particular, tests conducted on a prototype of the vehicle have shown that the sound level emitted by the pump exceeds the allowable limit by approximately 10 dBa.
To reduce the noise level of the pump, a first known solution consists in slightly tilting the plate along a second axis perpendicular to the first tilt axis in order to smooth the pressure peaks in the pistons and to reduce the sound excitation. However, this solution only provides a reduction of approximately 3 to 4 dBa of the overall noise of the hydraulic pump and is not sufficient.
Another known solution to reduce noise emissions consists in encapsulating the hydraulic pump. This solution entails a significant space problem for the encapsulation, making it difficult to implement.